Digitizer tablets are well known input devices for computers. A typical construction employs planar electrodes embedded in the tablet surface just below its working surface cooperating with a pointing device on the surface. The pointing device may be in the form of a stylus, pen or cursor, and may contain a coil or similar device and frequently button switches. For tracing drawings accurately, the cursor is preferred. It has the general appearance of an elongated mouse, with a projecting transparent plate in front (sometimes in the center) containing a crosshair surrounded by a coil or similar device. In a typical use, the electrodes are activated generating electromagnetic fields that induce signals in the coil. The signals when processed output data representative of the X and Y coordinate positions of the coil center on the tablet surface. The accuracy of the data is in part determined by the perfection of the coil and coincidence of the crosshair with the coil center. The user uses the crosshair to trace the drawing or other pattern desired, and the data input to the computer depends on the alignment of the coil and crosshair.
A common way to manufacture the cursor is to premold a transparent plastic plate, to serve as the base for the housing and switches, and as a support for the coil and crosshair. The premold includes a circular through-hole. The coil is wound in a flat pancake form on a double-sided bobbin, and then placed in the hole so that one bobbin side lies in the same plane as the plate bottom. The bobbin has a depth approximately equal to that of the hole. The crosshair is preformed on one side of the bobbin. The bobbin is glued in place after it is manually rotated so that the crosshair is exactly in line with grooves formed in, and extending along the longitudinal axis of, the plate.
This known method exhibits several drawbacks. The first is cost. The manual assembly and alignment of crosshair with plate grooves is relatively expensive. The second drawback is that exceptional care is necessary to ensure that the mechanical center of the crosshair matches the electrical center of the coil. A third drawback is that the level of electrostatic discharge resistance (ESD) while adequate could be enhanced.